Effect of screw access channel
filling materials on uniaxial
retentive force of cement-retained implant restorations
U Cakan,* P Gultekin,
† MB Guncu,‡ S Canay‡
*Department of Prosthodontics, Faculty of Dentistry, Istanbul Medipol University, Istanbul, Turkey. †Department of Prosthodontics, Faculty of Dentistry, Istanbul University, Istanbul, Turkey. ‡Department of Prosthodontics, Faculty of Dentistry, Hacettepe University, Ankara, Turkey.
ABSTRACT
Background: This study aimed to investigate the effect of various abutment screw access channel filling materials on the uniaxial retention of castings cemented onto the abutment using temporary cement.
Methods: Ten straight, regular platform Esthetic Abutments were used. Fifty castings were divided into five groups and the abutment screw access channels were filled with composite resin (Filtek Z 250), light-cured temporary filling (Clip), temporary filling (Coltosol), polyvinyl siloxane impression material (Elite H-D), or polytetrafluoroethylene (PTFE) tape. Castings were cemented and thermal cycled. A uniaxial tensile force with a cross-head speed of 5 mm/min, was applied to the castings until cement failure occurred. One-way analysis of variance was used to perform intergroup comparisons of the mean uniaxial retentive force (URF) values and Tukey’s HSD test was used to determine the group causing the difference.
Results: There were statistically significant differences between the mean URF values for the groups (p< 0.01). The mean URF value for the composite resin group was statistically higher than those for the other filling material groups respec-tively (p< 0.05).
Conclusions: The retention of castings cemented to straight implant abutments using non-eugenol temporary cement may be influenced by the screw access channel filling material.
Keywords: Implant, abutment, uniaxial retention, screw access channel, cement.
Abbreviations and acronyms: ANOVA = analysis of variance; PTFE = polytetrafluoroethylene; PVS = polyvinyl siloxane; URF = uniaxial retentive force.
(Accepted for publication 22 April 2013.)
INTRODUCTION
During recent decades, prosthetic rehabilitation of edentulism with dental implants has become a scientifi-cally well-documented and commonly established treatment modality. Implant-supported restorations often represent a better alternative than conventional treatment modalities for tooth replacement. The selec-tion of the retenselec-tion method, screw-retained or cement-retained, involves a complex and comprehensive deci-sion including consideration of the many points.1,2
Cement-retained, implant-supported restorations offer several advantages over screw-retained restora-tions, including enhanced aesthetics, with the elimina-tion of screw access holes; greater resistance to fracture; more stable occlusal contacts in the area of the screw access channel; shorter and fewer appoint-ments for fabrication; and comparatively easy and cost-effective laboratory procedures.3–6 However, in
circumstances involving the existence of deep submu-cosal shoulders, failure to remove excess cement is a major disadvantage that may lead to inflammatory tis-sue responses and/or bone loss.7 Modern implant sys-tems have new designs, involving custom abutments, which position the cemented junction either slightly subgingivally or supragingivally, thus reducing com-plications.
Retention is a key factor for determining the clinical success of cement-retained implant-supported restora-tions.5 The factors that influence the retention of cement-retained restorations have been well docu-mented. Cement type, composition, variations in cement viscosity, film thickness, variations in the seat-ing forces, the duration of the force applied, as well as the axial wall taper, diameter, height, surface area, and roughness of the abutments have been shown to impact the retention of implant-supported restora-tions.8–10
Australian Dental Journal 2014; 59: 65–69 doi: 10.1111/adj.12148
The fabrication procedure of implant supported res-torations in many ways resembles the fabrication of tooth supported fixed partial restorations where the presence of a screw access channel is the major differ-ence between a prepared natural tooth and an implant abutment onto which a crown may be cemented.
Materials such as cotton, gutta-percha, polyvinyl siloxane (PVS), auto-polymerizing acrylic resin, com-posite resin, temporary restorative fillings, and poly-tetrafluoroethylene (PTFE) tape, have been suggested for partial or complete filling of the screw access channels prior to cementation of the restoration.11–13
These materials cover the head of the abutment screw, but enable future clinical access to the screw if required. Subjective factors, such as the ease of mani-pulation, influence a clinician’s preference of filling material.
The factors influencing the retention of dental resto-rations involving implant abutments have been thor-oughly investigated in the literature.5–8 Even though the screw access channel comprises a relatively smaller portion of the abutment in contact with the cement for short abutments, the screw access channel filling material may have an effect on retention. Therefore, comparison of the filling materials may have clinical relevance in terms of selection of the correct material.
A limited number of studies assessing the efficacy of screw access channel filling materials on retention have been performed on angled implant abut-ments.14,15 However, the presence of a correlation
between the different filling materials used with short straight abutments and their retention has not previ-ously been investigated. The null hypothesis of this study was that varying screw access channel filling materials would not have a significant effect on the uniaxial retention force (URF) of castings cemented to short straight implant abutments.
MATERIALS AND METHODS
Ten regular platform, straight Esthetic Abutments (Branemark System®, Nobel Biocare, G€oteborg, Sweden) were attached to their implant replicas with a 35 Ncm torque by using a torque control device. Implant assemblies were mounted vertically into auto-polymerizing acrylic resin (Orthocryl 2000®; Denta-rum, Pforzheim, Germany) blocks with a surveyor
using a custom, prefabricated, stainless steel aligning tip inserted into the abutment screw access channels. The minimum abutment height for fabrication of a cement-retained implant supported crown has been reported to be 5 mm.16 Aesthetic abutments were cut horizontally to a height of 5 mm with a wire electro-discharge machine (Sodick A500; Fine Sodick Mark XI EDW, Sodick, Japan) and the height of each abut-ment was calculated and verified using a measuring microscope (Model MM-11U; Nikon, Melville, NY, USA). The screw access opening of each abutment was filled with polyvinyl siloxane putty (Elite H-D; Zhermack, Rovigo, Italy). Two layers of die spacers (Peel-Away Die Spacer, J Morita, CA, USA) were applied to the abutments within 1 mm of the margin. A total of 50 wax patterns with occlusal attachment rings were fabricated. The wax patterns were sprued, invested, and cast in nickel chrome ceramic alloy (Delta Ceram, Tritech, Voerde, Germany). After divestment, inner and outer surfaces of the castings were airborne-particle-abraded with 50lm aluminium oxide (Ivoclar® Vivadent, Amherst, New York, USA) and adjusted onto the individual implant abutments with a fit checker (Fit Checker White, GC America Inc., Alsip, IL, USA). Each casting was numbered for easy identification during testing and assigned to cor-respondingly numbered abutments. Fifty casting-abut-ment assemblies were randomly divided into five groups (n = 10), and in each group, the screw access channels of the abutments were filled with different materials. These materials and their compositions are listed in Table 1. One prefabricated cotton pellet (Cotton pellets 4.0, Roeko, Langenau, Germany) was placed into each abutment screw access channel and the remaining space in the channel was then com-pletely filled with the selected material using a hand instrument within the contour of the abutment.
Weighed amounts of temporary non-eugenol cement (Temp Bond, Kerr Corp, CA, USA) were mixed, according to the manufacturer’s recommendations, and applied to the fitting surface of the castings. Each casting was placed with finger pressure for five sec-onds. The specimens were then subjected to a 5 kg load for five minutes.8,14,15 Excess cement was
removed using a scaler. Mixing and cementing
proce-dures were performed at room temperature (24
2°C) by the same investigator. Specimens were stored Table 1. Screw access channelfilling materials used in this study
Product Material type Manufacturer
Filtek Z 250 Universal restorative composite resin 3M ESPE, St Paul, MN, USA
Clip Light curing temporary filling Voco, Cuxhaven, Germany
Coltosol Temporary filling material Coltene,Whaledent, Switzerland
Elite H-D Polyvinyl siloxane putty Zhermack, Rovigo, Italy
at 37°C for 24 hours and thermal cycled for 5000 cycles, at 5–55 °C with a 30-second dwell time. After thermal cycling, each assembly was attached to a uni-versal testing machine (Autograph AG-X, Shimadzu Corp., Kyoto, Japan) to apply a tensile force with a crosshead speed of 5 mm/min.8,14,15 The mean URF values in Newtons were recorded. The modes of cement failures, either adhesive or cohesive were also evaluated.
During the assessment of the data obtained in the study, SPSS (SPSS, Chicago, IL, USA) for Windows, version 15.0, was used for statistical analysis. One-way analysis of variance (ANOVA) was used to inves-tigate the intergroup comparisons of parameters with-out normal distributions and the Tukey’s HSD test was used to determine the group causing the differ-ence. Significance was evaluated at a level of p < 0.05.
RESULTS
The mean URF values related for the five different filling materials are shown in Table 2. One-way ANOVA showed there were statistically significant differences between the mean URF values for the vari-ous groups (p< 0.01) (Table 2). The mean URF value of the Filtek Z 250 group was statistically higher than those for the Clip, Coltosol, Elite H-D and PTFE tape groups respectively (p< 0.05) (Fig. 1). The Clip group presented statistically higher values than the Elite H-D group (p< 0.01), and the difference between the Clip and Coltosol groups was statistically insignificant (p> 0.05) (Table 3). The Coltosol group presented a sta-tistically higher URF value than the PTFE tape group (p < 0.01). A statistical difference was not observed between the mean URF values for the Elite H-D and PTFE tape groups (p > 0.05) (Table 3). The cement failure occured adhesively at the temporary cement-titanium abutment interface for 83% of the implant-casting assemblies. Cement was found mostly on the fitting surface of the castings, while for Filtek Z 250, Clip and Coltosol groups the temporary cement had adhered to the filling material surfaces. For Elite H-D and PTFE tape groups, cement residue was found
mostly on the fitting surface of the castings and very little on the filling material surfaces.
DISCUSSION
The null hypothesis of this study stated that varying the screw access channel filling materials would not have a significant effect on the mean URF values of the castings cemented to short implant abutments; the null hypothesis was rejected. Varying the screw access channel filling materials had significant effects on the URF values of the cement-retained, implant-supported restorations. The mean URF values of the studied materials were ordered as follows: Filtek Z 250> Clip > Coltosol > Elite H-D > PTFE tape.
It has been reported that various materials have been used to fill the screw access channels of the abut-ments of cement-retained or screw-retained implant-supported restorations.11–13 In the present study, the most retentive filling material, a composite resin (Filtek Z 250), is easy to manipulate but presents a risk of damaging the head of the abutment screw dur-ing removal.
Table 2. Mean uniaxial retentive force (URF) values for the differentfilling materials
Material URF (Newton) p
Mean SD Filtek Z 250 118.51 21.65 0.001** Clip 96.59 15.68 Coltosol 74.94 24.08 Elite H-D 53.84 9.57 PTFE tape 45.12 11.46
One-way ANOVA test** p < 0.01.
Elite H-D 25,00
Filtek Z 250 Clip Coltosol PTFE Tape 50,00 10 75,00 125,00 URF§ (Newton)
§Uniaxial retentive force
100,00
Fig. 1 Graphic presentation of median uniaxial retentive force values for differentfilling material groups.
Table 3. Post hoc test results
Material p
Filtek Z 250 / Clip 0.050*
Filtek Z 250 / Coltosol 0.001**
Filtek Z 250 / Elite H-D 0.001**
Filtek Z 250 / PTFE tape 0.001**
Clip / Coltosol 0.058
Clip / Elite H-D 0.001**
Clip / PTFE tape 0.001**
Coltosol / Elite H-D 0.068
Coltosol / PTFE tape 0.003**
Elite H-D / PTFE tape 0.796
PTFE tape, which presented the lowest URF value, is a relatively new material that has been used in den-tistry as a liner for trial seating; for easy removal of intracoronally retained restorations prior to cementa-tion; to repair abutment teeth under pre-existing crowns; as a block-out material for the connection of the locator and ball attachment components; and as a matrix to prevent etching and/or bonding of adjacent teeth.11,17,18PTFE is also a promising material for use in filling screw access channels to protect the head of the abutment screw in implant-supported prostheses, allowing easy manipulation as well as removal when required.
Cement-retained, implant-supported restorations offer better aesthetic results with the elimination of screw access holes and also allow the use of standard fixed partial prosthetic procedures in most situations. On the other hand, retrievability is a major drawback for the use of cement-retained restorations.7 Neverthe-less, although the screw-retained crown is retrievable, removing a well-fitting, cement-retained restoration when a complication occurs, can be a challenge, regardless of the cements used.
Although not a clear assumption, if retrievability is a major concern and the retentive form of the abut-ment is satisfactory, removal of a temporarily cemen-ted restoration may be easier when the screw access channels are filled with Elite H-D or PTFE tape.
In the present study, the shortest abutment height, 5 mm, indicated for the fabrication of a cement-retained, implant-supported crown was selected.16The present research hypothesis was to evaluate the poten-tial effect of the screw access channel filling material on retention, where the retentive form was compro-mised due to the minimum abutment height. The results of this study suggest that filling the screw access channel with Filtek Z 250 or Clip prior to temporary cementation may contribute to retention when the retentive form of the abutment is compromised.
After decementation, the modes of cement failure were recorded. The non-eugenol temporary cement was mostly found adherent to the filling material sur-faces in the Filtek Z 250 and Clip groups. This result is similar to the results of a previous study.14 In that
study, the effect of the screw hole filling method on the retention of implant-supported crowns, cemented on 15° angled abutments, with non-eugenol tempo-rary cement, was investigated. That study also reported that the bond strength was greatly reduced when composite core samples were cemented with eugenol temporary cement, to cast crowns where the eugenol, a phenol derivative, reacted with free radicals and thereby inhibited the polymerization of the resin system.18,19 In the present study, for the Coltosol, Elite H-D, and PTFE tape groups, cement was not found to be adherent to the filling material surfaces.
In accordance with Chu et al.,14 the temporary
cement adhered to the relatively rougher surfaces of the castings with no cement found on the prefabri-cated, polished, titanium abutment surfaces.
PVS impression materials are based on the inher-ently hydrophobic polydimethyl siloxane polymers.19 Takahashi and Finger demonstrated that in order to increase the relative affinity of a liquid for a PVS impression material, an extrinsic surfactant should be applied before pouring. PTFE is also a high-molecular-weight, hydrophobic compound, consisting of carbon and fluorine.20 Neither water nor water-containing substances wet PTFE, as fluorocarbons demonstrate dispersion forces due to the high electronegativity of fluorine.17 The lowest URFs of Elite H-D and PTFE
tape, may be attributed to the incompatibility of the water-containing temporary cement and the hydropho-bic nature of these materials.
In the present study, factors such as cement type, axial wall taper, diameter, height and surface area of the abutments were standardized in order to evaluate only the effect of the filling materials on retention. The size of the screw access channel is smaller than the axial surface of the abutment, in contact with the cement and thus the contribution of this smaller area to the retention may be assumed to be negligible for unprepared standard height straight abutments. How-ever, for shorter abutments, the effect of the screw access channel filling material on retention should be taken into consideration as increases occur in the rela-tive ratio of the screw access channel area to the axial abutment surface area. In agreement with this assump-tion, the results of the present study revealed that dif-ferent filling materials may affect the URF values of implant-supported restorations cemented on short straight abutments. Therefore, a more retentive filling material may be preferred for temporary cementation.
There are a number of limitations in this study. The effect of the filling materials was investigated under circumstances involving only one type of temporary cement, with the same abutment height and platform size. Another drawback of the study is the use of URF for evaluation of decementation. In a clinical situa-tion, restorations in the oral environment are sub-jected to additional functional or parafunctional forces that may also contribute to crown decementa-tion. Most luting cements, except the resin based ones, are prone to tensile failure due to brittle struc-ture.21 It has been reported that laterally directed forces may be more destructive to cement that is prone to tensile failure.22 The uniaxial retention test procedure used in the present study was planned to minimize off-axis forces which could vary the cement failure load and account for some of the scatter recorded. Also, the tensile test was used in our study to allow comparisons with previous studies.
The influence of additional factors, such as cement type, composition, variations in cement viscosity, film thickness, as well as abutment characteristics, includ-ing axial wall taper, diameter, height, surface area, and roughness were not investigated in this study. Therefore, further clinical studies are needed to con-firm our results by comparing more luting cements, varying abutment properties, and imitating the intra-oral conditions with improved test methods.
CONCLUSIONS
Within the limitations of this study, the following conclusions were drawn: (1) The material used to fill the screw access channels of the short straight abut-ments may have an effect on the retention of tempo-rarily cemented implant-supported restorations; (2) Removal of the temporarily cemented implant-sup-ported restorations may be easier when the screw access channels are filled with PVS or PTFE; (3) Fill-ing the screw access channel with Filtek Z 250 or Clip may contribute to retention of temporarily cemented restorations where the retentive form of the abutment is compromised.
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Address for correspondence: Dr Umut Cakan Istanbul Medipol University Department of Prosthodontics Faculty of Dentistry Atat€urk Bulvarı No. 27 34083 Unkapanı Fatih, Istanbul Turkey Email: ucakan@medipol.edu.tr
